A vehicle-travel control apparatus is known in the art, for example, as disclosed in Japanese Patent Publication No. 2009-051310, according to which a vehicle acceleration in a vehicle traveling direction is automatically controlled at a desired value. According to the vehicle-travel control apparatus of the above Patent Publication, generation of slip is surely suppressed to thereby automatically drive a vehicle. In order to achieve an automatic vehicle drive, an allowable torque which would not cause a wheel slip is calculated based on a ground load of a wheel as well as a coefficient of friction of a road surface. Furthermore, a limiting acceleration, which would be acted on the vehicle when the calculated allowable torque is applied to the vehicle wheel, is calculated. And a feed-forward torque, which would correspond to an acceleration actually acted on the vehicle, is calculated based on a comparison between the limiting acceleration and a demand acceleration.
The ground loads of the respective wheels differ from the wheel to wheel depending on a slope of the road surface. For example, in case of an uphill slope, the ground loads of front wheels become smaller, while the ground loads of rear wheels become larger. On the other hand, in case of a downhill slope, the above relationship is reversed. Therefore, the ground loads of the respective wheels are calculated by taking dynamic load distribution calculated from the slope of the road surface into consideration, so that the automatic vehicle travel can be done depending on various conditions of the road surface.
The above feed-forward torque is calculated by adding to each other a demand torque corresponding to the demand acceleration and an estimated-slope torque included in the ground load corresponding to the slope of the road surface. Therefore, in a case that calculation accuracy for the estimated-slope torque is decreased for some reason, accuracy for the feed-forward torque may be correspondingly decreased.